Women with breast cancer who take common blood pressure drugs may have better odds of surviving the disease, according to two preliminary studies.
In one study, women taking drugs known as beta-blockers survived longer without seeing the tumor return than those not on the medication. In the other, they were less likely to be diagnosed with aggressive breast cancer or die from it.
Still, the researchers strike a cautious note and say no one should consider taking beta-blockers to stave off cancer at this point.
"We saw an association, now it's time to prove whether they are the cause," said Dr. Amal Melhem-Bertrandt, who worked on one of the studies. "It's very promising, it's encouraging, but we still have to do the studies."
Beta-blockers curb the effect of stress hormones like adrenaline and noradrenaline, and are used to treat high blood pressure.
Cancer researchers began to take an interest in them after animal studies showed stress responses are linked to tumor growth.
"There is a lot of literature suggesting chronic stress may influence breast cancer recurrence," Melhem-Bertrandt told Reuters Health. "We wanted to see whether blocking one of the arms of your stress response would help reduce breast cancer."
So she and her colleagues looked at medical records for some 1,400 women treated for breast cancer with chemotherapy and surgery at the M.D. Anderson Cancer Center in Houston. About seven percent of the women also happened to be taking beta-blockers.
The doctors who examined the tumors after the women had surgery found no differences between those who were on beta-blockers — mainly metoprolol and atenolol — and those who weren't.
However, the women on beta-blockers did seem to fare better afterward. At three years, 87 percent of them were alive and cancer-free, compared to 77 percent of those not taking the drugs.
Results were better with beta-blockers even after the researchers accounted for differences in age, cancer stage, diabetes and other factors that might influence tumor growth.
And the findings were even more striking for women with so-called triple-negative breast cancer, which doesn't respond to hormone therapy.
"We used to think of these drugs as innocent bystanders, but it looks like they may have an effect on cancer itself," said Melhem-Bertrandt.
Still, hidden factors could be at play, such as lifestyle or medication differences between the women. So it's too soon to say anything about cause and effect, Melhem-Bertrandt explained.
What's more, her study wasn't large enough to show a potential effect on overall survival.
In recent work from the U.K., researchers found similar survival rates among breast cancer patients taking beta-blockers and those taking other blood pressure medications.
"Larger studies are needed to clarify the effect of beta-blockers on breast cancer outcomes," Dr. Sunil Shah, who worked on the UK study, told Reuters Health in an email.
"But, if these benefits are confirmed, these findings are potentially important for a sub-group of women with breast cancer as beta-blockers are a relatively safe and inexpensive therapy," added Shah, of St George's University of London.
Beta-blockers can be bought for less than $10 per month.
The other new study, published along with Melhem-Bertrandt's results in the Journal of Clinical Oncology, tapped into data from a cancer registry and a pharmacy database in Ireland.
Women with breast cancer who were taking a type of beta-blocker called propranolol were much less likely to be diagnosed with advanced breast cancer than closely matched patients not on the drugs, according to Dr. Thomas Barron from Trinity Center for Health Sciences in Dublin, Ireland and colleagues.
Women taking propranolol also fared better after being diagnosed with the disease: After five years, an estimated nine percent had died of breast cancer, compared to 27 percent of women not on propranolol.
There was no such difference for atenolol, another beta-blocker, however. That contradicts Melhem-Bertrandt's findings, and suggests not all beta-blockers work the same.
With this much uncertainty, Melhem-Bertrandt said, beta-blockers aren't ready for prime time yet in the breast cancer world.
"They do have side effects," she told Reuters Health. "They can drop your blood pressure, they can slow your heart rate. I would not recommend women go on beta-blockers as a preventive measure."